Cisco Router Summarization

Learn the why and how for route summarization.

Category:

Education

Tags:

• Cisco
• CCNA
• IP
• Address
• summary route
• summarization
• VLSM

[arifsali]

Hi Keith, thank you so much for this.

I just wish to make sure and confirm: from the left, you "add" the bits which are matching among all routes (1 0 0 is matching and it adds up to 128 in this example) and that would be part of the summary route? If you had 1 1 as a common bit among all routes, that would be 192. So on and so forth. If this is correct, than that's beautiful.

But I am still struggling to figure out how you arrived at the subnet mask of 27?

[Keith6783]

@arifsali

You are right on the math.

100 if those were the 3 high order common bits, it would be 128

101 if those were the 3 high order common bits, it would be 160

etc.

Best wishes,

Keith

[miszczpolska]

Lets take as an example the range of IPs 172.16.128.0-172.16.159.255 We see that the network mask will have to affect 3rd octet. The easy way it to subtract 128 from 159. Lets see 159-128=31 Now subtract 31 from 255 255-31=224 There we go, this is our 3rd octet in the subnet mask 172.16.128.0 255.255.224.0 Another example Summarize networks 100.16.0.0 100.17.0.0 100.18.0.0 100.19.0.0 19-16=3 255-3=252 Our subnet mask is 255.252.0.0 for network 100.16.0.0

[Keith6783]

@miszczpolska

Thanks for the great additional information!

Best wishes,

Keith

[MadProd209]

Hi Keith,

Great video so one question what about if I add another router outside of the range of 23.1.2.128 - 23.1.2.159. do I need to recalculate the best summary or just advertise that new nework?

best regards,

Alcides

[Keith6783]

@MadProd209

If it is just one additional network, outside the range, just letting the route be advertised is our only real option (with only a single additional route). If we had many new routes that were close to each other, we could summarize those new routes into a matching summary.

With R9, because he is a stub, we could really just use a default static route on R9, and we would be done as well.

Great question, and thanks for posting!

Keith

[rameunknown]

This was an awesome explanation. Keep it up buddy.

[arifsali]

Oh, oh, oh, never mind, I think I just understood where 27 is coming from. The 27 represent x (8). x (16). x (24) and 1 0 0 (27). :)

[hedgeduck1]

you're intro drives me crazy

[miszczpolska]

@Keith6783 thanks!. I am studying for CCNA and network summarization was confusing me a bit. I figured it out similar way as with subnetting and finding the block size, etc /29... 32-29=3 (the power). 2^3=8 =block size. Or /29 is 255.255.255.248. Subtract 248 from 255 gives 8 as well, but easier way.